1. Field of the Invention
The present invention relates to a photovoltaic device, such as a solar cell, realizing high performance and high reliability over a prolonged period, and enabling easy mass production, and, more particularly, to an improved metal layer provided on the photovoltaic device.
2. Related Background Art
As energy sources in the future, total reliance on the fossil fuels, use of which is said to lead to the warming of the earth due to the generation of carbon dioxide, or on atomic energy, which inevitably involves the possible danger of radiation in an unpredictable accident or even in normal operation, involves various problems. For this reason, widespread use has been expected for solar cells which utilize the unlimited energy of the sun and which have only very limited influence on the ecology of the earth. At present, however, such wide use is being prevented by certain drawbacks.
For solar power generation, there has principally been employed monocrystalline or polycrystalline silicon. However, such solar cells are difficult to mass-produce and cannot be provided inexpensively, as they require much time and energy for crystal growth and also complex subsequent steps. On the other hand, much developmental activity has been conducted on solar cells capable of easier mass production, such as those employing amorphous silicon (hereinafter written as a-Si) or so-called thin film semiconductor solar cells employing compound semiconductors, for example, CdS or CuInSe.sub.2. Such solar cells offer the possibility of cost reduction, as they can be prepared by forming the necessary semiconductor layers on an inexpensive substrate such as a glass or stainless steel sheet through relatively simple manufacturing steps. However, such thin film solar cells have not been employed widely as they are inferior in conversion efficiency to the crystalline silicon solar cells and are not satisfactory in reliability during prolonged use. For this reason, there have been proposed various methods for improving the performance of the thin film solar cells.
For example, for improving the conversion efficiency, it is proposed to form a rear reflection layer in order to increase the light reflectance on the substrate surface, thereby returning the solar light not absorbed in the thin semiconductor film to the film for achieving effective utilization of the incident light. For this purpose, in the case where the photoelectric conversion layer is formed on a transparent substrate through which the solar light is introduced, the electrode formed on the outer surface of the thin semiconductor layer is preferably formed of a metal with a high reflectance, such as silver (Ag), aluminum (Al), or copper (Cu). Also, in the case where the solar light is introduced from the outer surface of the thin semiconductor film, a similar metal layer is preferably formed on the substrate, prior to the formation of the thin semiconductor film constituting the photoelectric converting layer. A further improvement in the reflectance can be achieved by multiple interference, by forming a transparent layer of a suitable optical property, between the metal layer and the thin semiconductor film. FIGS. 3A, 3B show the result of simulation, indicating that the presence of a transparent zinc oxide (ZnO) layer between silicon and various metals (FIG. 3A) improves the reflectance, in comparison with the configuration without such zinc oxide layer (FIG. 3B).
The use of such transparent layer is also effective in improving the reliability of the thin film solar cells. The Japanese Patent Publication No. 60-41878 discloses that the presence of such transparent layer prevents alloy formation between the semiconductor and the metal layer. Also, the U.S. Pat. Nos. 4,532,372 and 4,598,306 teach that the use of a transparent layer of a suitable resistance can prevent generation of an excessive current even if a short circuiting occurs in the semiconductor layer.
Another method for improving the conversion efficiency of the thin film solar cells consists of forming a fine irregular structure (textured structure) on the surface of the solar cell and/or at the interface with the rear reflection layer. With such structure, the solar light is scattered at the surface of the solar cell and/or at the interface with the rear reflection layer and is enclosed in the semiconductor (light trapping effect), thereby being effectively absorbed therein. In the case where the substrate is transparent, the irregular structure is preferably formed on the surface of a transparent electrode, such as of tin oxide (SnO.sub.2) formed on the substrate. In the case where the solar light is introduced from the surface of the thin semiconductor film, the irregular structure is preferably formed on the surface of the metal layer constituting the rear reflection layer. M. Hirasaka, K. Suzuki, K. Nakatani, M. Asano, M. Yano and H. Okaniwa (Solar Cell Materials, 20(1990), pp. -110) reported that the irregular structure for the rear reflection layer could be obtained by aluminum deposition with regulation of the substrate temperature and the deposition rate. FIGS. 4 and 5 show examples of an increase of absorption of the incident light, by employing such irregular surface structure in the rear reflection layer. In FIG. 4, curve (a) indicates the spectral sensitivity of an a-Si solar cell employing a smooth Al layer, while curve (b) indicates that of an a-Si solar cell employing an Al layer with irregular surface structure. In FIG. 5, curve (a) indicates the spectral sensitivity of an a-SiGe solar cell employing a smooth Cu metal layer, while curve (b) indicates that of a similar cell employing a Cu layer with irregular surface structure.
It is also possible to combine the concept of the rear reflection layer consisting of a metal layer and a transparent layer, with the concept of the irregular surface structure. The U.S. Pat. No. 4,419,533 discloses the concept of the rear reflection layer in which the surface of a metal layer has irregular structure and a transparent layer is formed thereon.
Use of aluminum or copper as the metal layer of the rear reflection layer is extremely useful in obtaining a solar cell with a low manufacturing cost and a high conversion efficiency. However, the conventional rear reflection layer in which the metal layer or a substrate serving as the metal layer and composed of aluminum or copper cannot necessarily provide sufficient reliability in use under harsh conditions such as high temperature and high humidity, resulting eventually in a significant loss in the conversion efficiency. For this reason, such thin film semiconductor solar cells have not been commercially employed for solar power generation, though they offer the possibility of inexpensive manufacture.